Such drying plants for structural panels are known in the form of convection driers. The panels to be dried pass on a conveyor belt sequentially into the convection drier, wherein the panels are exposed to heated air on both surfaces of the panel. The air temperature may change from the drier input end to its output end. These temperatures are within the range of about 80.degree. C. to about 240.degree. C., whereby the undesirable water content of the panels is removed by these relatively high air temperatures.
Depending on the thickness of the panels, and on other physical conditions, such as the temperature of the drier, the air speed, and so forth, the required drying times may be within the range of a few minutes up to an hour and more, especially where thicker panels are involved. Where the panel thickness exceeds 30 mm, the drying times become disadvantageously long. For example, fiber or chip reinforced sheetrock having a thickness of 38 mm requires a drying time of up to 6 hours and more. Thus, assuming an hourly throughput rate of 130 m.sup.2 of panel surface, a four tier drier would be required, having a length between 80 and 90 m. Such a drier requires substantial capital investments, not only for the drier itself, but also for the building to house the drier. Another disadvantage of prolonged drying times with the panel material exposed to higher air temperatures, is the fact that especially the surface of the panel may be damaged, for example, due to dehydrating the gypsum.
There are also high frequency driers available in the art for drying by water withdrawal. These driers have so far been used in the paper production for a preliminary drying of paper and cardboard products. Such high frequency driers have also been used for a complete drying of such products, whereby however a substantial power input is required per volume of paper product dried. As a result, such high frequency driers have not been very economical as far as their operating costs are concerned. Another disadvantage of high frequency driers or ovens has been in the past that the vapor exit could damage the product, especially if the product had larger thicknesses. In addition, or instead of the vapor exit damage, bubble formation was a problem due to the internally produced heat of the material to be dried. Even minor damages in limited areas are undesirable. For these reasons, high frequency driers have so far not been used for drying flat structural components, such as sheetrock. Rather, such high frequency driers have been used for thin thickness products, especially paper or the like.